Apparatus and method for improving the accuracy of a loss-in-weight feeding system

ABSTRACT

A control for loss-in-weight feeders which decreases flow error during refill of the feeder hopper. The weight range (empty to full capacity) of the hopper is divided into weight increments. Samples are taken of the hopper weight during discharge and refill, and each sample is assigned to a weight increment. During discharge, a scale factor related to the density of material in the hopper is computed, assigned to a weight increment and stored in memory; and the hopper feed screw is driven at a speed proportional to the computed scale factor. During refill, the stored scale factors are retrieved from memory and the hopper feed screw is driven at a speed proportional to the stored scale factors; but no additional scale factors are calculated.

This is a continuation of application Ser. No. 343,143 filed Jan. 28,1982 now U.S. Pat. No. 4,524,886.

BACKGROUND OF THE INVENTION

Prior art loss-in-weight feeders suffer from a loss in feed rateaccuracy during the refill cycle. This is believed to be causedprimarily by material compaction in the feeder hopper as material isadded to the hopper.

In a typical prior art loss-in-weight feeder, material having knowndensity is introduced at a relatively rapid rate into the feeder hopper.The material is then exhausted from the hopper at a much slower rate.The period during which material is introduced into the hopper isreferred to hereafter as "refill" or "refill cycle". The period duringwhich material is exhausted from the hopper is referred to as"discharge" or "discharge cycle". A feed screw located at the bottom ofthe hopper is rotated at a speed so as to deliver material at a desiredflow rate. At the very beginning of the refill cycle, when very littlematerial is in the hopper, the mass flow rate will be correct since thedensity of material within the hopper will be within expected limits. Asmaterial is added to the hopper, the material near the bottom of thehopper compacts. This increases the effective material density, and themass flow rate increases, producing a flow rate error.

SUMMARY OF THE INVENTION

The present invention is an apparatus and method for improving theaccuracy of a loss-in-weight feeding system during refill. Duringdischarge and refill, the weight of material in the feeding systemhopper is periodically sampled, and a signal representative of theweight sample is generated. The sample weight is assigned to one of aplurality of weight increments. Each weight increment is a fraction ofthe hopper weight range, from empty to full weight. The fraction is theinverse of the number of weight increments selected. During discharge, aplurality of scale factors is calculated and stored in memory. Eachscale factor is representative of the density of material in the hopperfor a weight increment corresponding to the sample weight. The scalefactors are repeatedly updated during successive discharge cycles.Between consecutive discharge cycles, the hopper is refilled. Duringrefill, the feed screw motor speed is varied in proportion to theupdated scale factors stored during a preceding discharge cycle.

This invention decreases the mass flow error during refill by drivingthe feed screw motor at a speed based on the varying density of materialin the hopper. Hopper weight measurements made during refill need not behighly accurate. Weight measurements need only be accurate enough todetermine what weight increment to select.

DETAILED DESCRIPTION OF THE DRAWINGS

For the purpose of illustrating the invention, there is shown in thedrawings a form which is presently preferred; it being understood,however, that this invention is not limited to the precise arrangementsand instrumentalities shown.

FIG. 1 is a simplified block diagram of a prior art loss-in-weightfeeding system.

FIG. 2 is a simplified block diagram of the loss-in-weight feedingsystem in accordance with the present invention.

FIGS. 3A and 3B are flow charts illustrating the operation of theloss-in-weight feeding system shown in FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings wherein like numerals indicate like elements,there is shown in FIG. 1 a simplified block diagram of a prior artloss-in-weight feeding system. Material is exhausted or discharged froma hopper by operation of a feed screw driven by a feed screw motor. Thespeed of the feed screw motor, and thus the mass flow rate (hereinafter"mass flow") of material discharged from the hopper, is controlled by amotor controller and associated motor drive in response to commandsignals from a programmed microprocessor. A desired or set point massflow is selected by means of numerical switches on an operator panel.The panel includes conventional circuitry which generates a mass flowset point signal based on the setting of the switches. The signalcomprises one input to the microprocessor. The other input to themicroprocessor is a weight signal from the weight processor. The weightsignal represents the current weight of material in the hopper. Thespeed of the feed screw motor is adjusted during discharge to delivermaterial at a substantially constant mass flow rate in response to theweight signal and the mass flow set point signal.

Problems develop during refill. When refill begins, the microprocessorchanges automatically from the mass flow control mode wherein the feedscrew motor speed is made proportional to mass flow error to thevolumetric flow control mode wherein the feed screw motor speed is heldsubstantially constant. In the volumetric control mode, the feed screwis operated to deliver a substantially constant volumetric flow ofmaterial. For an unchanging material density, this results insubstantially constant mass flow. But during refill, there is a changein material density due to compaction of material in the feeder hopper.The change in density of material in the hopper is not accounted for bythe prior art system. At a constant volumetric flow, this results inincreased mass flow error.

The present invention eliminates this problem by varying the speed ofthe feed screw motor, in the volumetric control mode, to compensate forchanging material density during refill.

There is shown in FIG. 2 a simplified block diagram of a loss-in-weightfeeding system 10 in accordance with the present invention. The systemincludes a conventional hopper feeder 12, a scale 14 and a drive motor16 for the feed screw (not shown) within the hopper 12.

Loop Control

The system 10 utilizes a programmed microprocessor 18 which provides athree loop feedback control system. The primary and fastest loop drivesthe feed screw motor at a speed proportional to the frequency of a pulsetrain signal generated by a rate multiplier 20. The input to the ratemultipler 20 is a digital word which is the product of the mass flow setpoint signal and a scale factor stored in memory 22. Computation of thescale factor is described in detail below. The mass flow set pointsignal is a digital word representative of the desired mass flow. Thesignal is generated in conventional manner at the output of an operatorpanel 24 based on an operator setting for thumbwheel switches 26. Theother input to the rate multiplier is a fixed frequency pulse traingenerated by a clock 28. The frequency of the rate multiplier pulsetrain output is the fixed frequency of the clock pulse train multipliedby the product of the scale factor and mass flow set point. The primaryloop is closed by a tachometer 30, which generates a pulse train signalhaving a frequency proportional to the feed screw speed, and an up/downcounter 32 which is shown as a hardware element external to themicroprocessor 18 but which may be part of the microprocessor softwareif desired. The up/down counter 32 counts up the rate multiplier outputpulses and counts down the tachometer pulses. The net count isrepresentative of mass flow error. The mass flow error signal generatedby the counter commands the motor drive 34 which generates the speedcommand signal for the drive motor 16.

The secondary loop, which is a multiplicative servo loop slower than theprimary loop described above, calculates and stores the scale factorswhich are used to compensate for mass flow error induced by compactionof hopper material during refill. A sampling circuit 36 periodicallysamples the weight signal generated by scale 14. The sample weightsignal generated by sampling circuit 36 is used to compute or update ascale factor and to address the memory 22 so as to store the scalefactor at the appropriate address. Storage of the scale factors at theappropriate addresses is described more fully hereafter. Duringdischarge, the scale factors are calculated or updated by a compute massflow circuit 38, a division circuit 40, and a multiplier circuit 42.Computation of mass flow is based on successive weight samples asdescribed more fully hereafter. Division circuit 40 calculates the ratioof mass flow set point to the computed mass flow. This ratio provides anindication of effective material density during a subsequent refillcycle for each weight increment in the hopper weight range. As will beexplained hereafter, the ratio calculation is made for each weightsample taken during discharge. An assign to weight range circuit 44assigns each weight sample to a weight increment. Each weight incrementcorresponds to an address of the scale factor memory 22. The calculatedor updated scale factor (termed "active" scale factor) is then stored inthe memory 22.

During each refill cycle, the scale factors are retrieved from memoryand sent to the rate multiplier circuit 20. Appropriate microprocessorlogic circuitry (not shown) is supplied to direct the system through theprimary and secondary loops as described more fully below in connectionwith the system flow charts in FIGS. 3A and 3B. The output of counter 32during each refill cycle is thereby compensated for changing materialdensity.

The third and slowest loop within the system minimizes long term error.The loop includes a compute mass flow error circuit 46 which computeslong term mass flow error, i.e. the difference between computed massflow and mass flow set point multiplied by time. The long term mass flowerror is added to the mass flow error generated in the primary loop.

The scale factor memory 22 is arranged to hold any desired number ofscale factors. Each scale factor is stored at the appropriate weightincrement address. At present, ten weight increments (hence ten scalefactors) are preferred as a reasonable value which provides suitableaccuracy, but any number other than ten may be selected.

System Operation

Details of the system operation are best understood by referring toFIGS. 3A and 3B. At the beginning of discharge, the mass flow set pointsignal, selected by manipulation of thumbwheel switches 26 at theoperator panel 24, is detected by microprocessor 18 (GET FLOW COMMANDKG/HR). The microprocessor then checks whether a predetermined weightperturbation has been sensed (WEIGHT PERTURBATION?). The predeterminedweight perturbation is sensed by computing the change in weight betweensuccessive weight samples. As described more fully below, an increase inweight beyond a predetermined limit is regarded as the weightperturbation (IS NEW WEIGHT READING WITHIN EXPECTED RANGE?), and aweight perturbation indicates a refill operation. If no weightperturbation has been sensed, this indicates that material is beingdischarged without refill.

During the discharge cycle, the program branches to the CALCULATE MOTORSPEED COMMAND block wherein the motor speed command is based onmultiplication of the "active" scale factor by the mass flow set pointand fixed frequency clock signal (rate multiplier 20). The feed screwmotor 16 is driven in response to the counter 32 output (OUTPUT MOTORSPEED COMMAND). Counter 32 counts up the rate multiplier output pulsesand counts down the tachometer pulses as previously explained.

If a weight perturbation has been sensed, indicating that the hopper isbeing refilled, the program enters the REFILL COMPENSATION routineindicated in dashed lines. The current weight sample is assigned to oneof the aforesaid weight increments (DETERMINE FROM CURRENT WEIGHT THEWEIGHT BRACKET NUMBER). For the assigned weight increment, thecorresponding scale factor is recalled from memory (USING WEIGHT BRACKETNUMBER AS INDEX GET VOLUMETRIC DENSITY FACTOR FROM VDF ARRAY) and thenmultiplied by the mass flow set point signal (CALCULATE MOTOR SPEEDCOMMAND). The product is used by rate multiplier 20 to modify thefrequency of the clock pulse train. The rate multiplier output pulsesare fed to the up terminal of pulse counter 32 (OUTPUT MOTOR SPEEDCOMMAND). The mass flow error signal generated by the counter determinesthe drive motor speed.

A new sample of the hopper weight is then taken (NEW WEIGHT SENSORREADING AVAILABLE?). If the new sample weight reading is not within anexpected predetermined range of the prior sample (IS NEW WEIGHT READINGWITHIN EXPECTED RANGE?), a weight perturbation is detected (ASSERTPERTURBATION) and the foregoing operations are repeated in the REFILLCOMPENSATION routine. If the new weight sample is within an expectedpredetermined range of the prior sample, no perturbation is detected(NEGATE PERTURBATION), indicating a discharge cycle rather than a refillcycle. The change between weight samples is computed (COMPUTE WEIGHTCHANGE), and the mass flow and long term mass flow error are computed(CALCULATE MASS FLOW RATE . . . AND LONG TERM MASS FLOW ERROR).

The mass flow error is computed by circuit 46 as the difference betweenthe mass flow set point and computed mass flow. If the mass flow erroris within a preset error band (IS MASS FLOW RATE ERROR WITHIN PRESETFLOW RATE ERROR BAND?), the error is not displayed (NEGATE MASS FLOWRATE ERROR) and the weight sample is assigned to a weight increment(DETERMINE WEIGHT BRACKET NUMBER). An "active" scale factor is thencalculated (CALCULATE NEW VALUE OF ACTIVE DENSITY FACTOR) and stored inmemory for that same weight increment in replacement of the previouslystored scale factor (PUT NEW VALUE OF ACTIVE DENSITY FACTOR INTO VDFARRAY . . . ). The "active" scale factor is updated to reflect thechange in material density due to the change in hopper weight. The"active" scale factor is then employed to generate the feed screw motorspeed command signal (CALCULATE MOTOR SPEED COMMAND), as alreadyexplained, until a new sample weight is available. Should the mass flowerror exceed the preset band for any weight sample during discharge, themass flow error is displayed (ASSERT FLOW RATE ERROR) and the computedmass flow is not utilized to calculate an "active" density factor sincethe data is no longer considered reliable. The scale factors stored inmemory remain as is.

From the foregoing, it should be appreciated that weight samples arerepeatedly taken and tested for a weight perturbation during a dischargecycle. Concurrently, scale factors stored in memory are repeatedly beingrecalled and replaced by "active" scale factors. During refill, however,the stored scale factors are recalled from memory and are used togenerate the motor speed command without replacing any of the scalefactors with "active" scale factors. An "active" scale factor is simplyan updated stored scale factor, being the product of the stored scalefactor (recalled from memory) and the ratio of mass flow set point tocomputed mass flow. Thus, the system employs an iterative techniquewherein scale factors are recalled from memory during discharge cycle,multiplied by the ratio of mass flow set point to computed mass flow toproduce an "active" scale factor, and replaced in memory by the "active"scale factor so computed. Between successive discharge cycles, thehopper is refilled. During each refill cycle, the stored scale factorsare recalled from memory and employed to generate the rate multiplieroutput, but new "active" scale factors are not calculated. An activescale factor, being based on a computation of mass flow set point tocomputed mass flow during discharge, is indicative of the changingdensity of material in the hopper for each weight increment in thehopper weight range.

It should be noted that the scale factor memory 22 may be initiallyloaded, at all addresses, with the same arbitrary numbers. For example,each address may contain the number 1,000. During the first dischargecycle, these numbers are recalled from memory and multiplied by theratio of the mass flow set point to computed mass flow to derive thefirst "active" scale factors. These "active" scale factors then replacethe numbers initially stored in memory. During subsequent dischargecycles, these "active" scale factors are recalled from memory andreplaced with new "active" scale factors by the iterative techniquedescribed.

An embodiment of the invention has been described in terms of a blockdiagram (FIG. 2) comprising certain circuits contained within amicroprocessor. It should be noted that these circuits are merelyrepresentative of functional operations performed by conventionalmicroprocessor internal circuity when the microprocessor is programmedas indicated by the flow charts depicted in FIGS. 3A and 3B. Equivalenthardware circuits could, however, be substituted for the programmedmicroprocessor without exceeding the spirit or scope of the invention.

The present invention may be embodied in other specific forms withoutdeparting from the spirit or essential attributes thereof and,accordingly, reference should be made to the appended claims, ratherthan to the foregoing specification, as indicating the scope of theinvention.

We claim:
 1. A material feeding system comprising:means for storing aquantity of material; means for discharging said material from saidmeans for storing during a first period and a second period, saidmaterial having an effective density which is dependent on the quantityof material in said storing means; means for generating a signalrepresenting the effective density of material being discharged duringsaid first period, said signal being different for different effectivedensities; and means responsive to the quantity of material in saidstoring means during said second period and to said signal forcontrolling said means for discharging during at least a portion of saidsecond period to compensate for change of the effective density of thematerial being discharged.
 2. A material feeding system comprising:meansfor storing a quantity of material; means for discharging said materialfrom said means for storing during a first period and a second period;means for generating and storing a signal related to the discharge ofmaterial during said first period, said signal being different fordifferent quantities of material stored; and means responsive to thequantity of material in said means for storing during said second periodand to said stored signal for controlling said means for dischargingduring at least a portion of said second period.
 3. A material feedingsystem comprising:means for storing a quantity of material; means fordischarging said material from said means for storing during alternatingfirst and second periods; means for refilling said means for storingduring said second periods; means for generating and storing a signalrelated to the discharge of material during said first period, thedischarge of material being different for different quantities ofmaterial stored; and means responsive to the quantity of material insaid means for storing during said second period and to said storedsignal for controlling said means for discharging during at least aportion of said second period.
 4. A material feeding systemcomprising:means for storing a quantity of material; means fordischarging said material from said means for storing during a first andsecond period; means for generating and storing a signal related to thedischarge of material during said first period, said discharge ofmaterial being different for different quantities of material in saidmeans for storing; and means responsive to the quantity of material insaid storing means during said second period and to a signal derivedfrom said stored signal for controlling said means for dischargingduring at least a portion of said second period.
 5. A material feedingsystem comprising:means for storing a quantity of material; means fordischarging said material from said means for storing during first andsecond periods; means for sensing the weight of said quantity ofmaterial; means for generating and storing a signal related to thedischarge of material during said first period and related to saidweight sensed during said first period; and means responsive to theweight of material in said storing means during said second period andto a signal derived from said stored signal for controlling said meansfor discharging during at least a portion of said second period.
 6. In aloss in weight feeding system including means for receiving a quantityof material and means for discharging said material during alternatingfirst and second periods wherein the means for discharging is controlledto provide discharge at a predetermined mass flow rate during said firstperiod and discharge at a volumetric flow rate during said secondperiod, the improvement comprising:means responsive to the weight ofmaterial during said first period for generating and storing a signalrelated to the discharge of material for a predetermined weight of thequantity of material; and means responsive to the weight of materialduring said second period and to a signal derived from said storedsignal for controlling said volumetric flow rate during said secondperiod.
 7. A method for feeding material comprising:storing a quantityof material; discharging said material from said stored quantity duringalternating first and second periods; generating and storing a signalrelated to the discharge of material during said first period; andcontrolling said discharge during at least a portion of said secondperiod in response to the quantity of material stored during said secondperiod and a signal derived from said stored signal.
 8. A method forfeeding material comprising:storing a quantity of material in a meansfor storing; discharging said stored material from said means forstoring during alternating first and second periods; sensing the weightof material discharging during said first period; computing the massflow rate of material being discharged during said first period inresponse to said weight sensed during said first period and maintainingsaid mass flow rate at a selected value during said first period;storing a signal related to the discharge of material during said firstperiod, said signal being different for different rates of discharge;refilling said means for storing during said second period; sensing theweight of material in said means for storing during said second period;and controlling said discharge during at least a portion of said secondperiod in response to said weight sensed during said second period andto a signal derived from said stored signal.
 9. A system as recited inclaim 1 wherein said signal is in digital form.
 10. A system as recitedin claim 2 wherein said signal is in digital form.
 11. A system asrecited in claim 3 wherein said signal is in digital form.
 12. A systemas recited in claim 4 wherein said stored signal is in digital form. 13.A system as recited in claim 5 wherein said stored signal is in digitalform.
 14. In a loss in weight feeding system as recited in claim 9wherein said stored signal is in digital form.
 15. A method as recitedin claim 7 wherein said stored signal is in digital form.
 16. A methodas recited in claim 8 wherein said stored signal is in digital form.